Problem 116 - A Complete Resource Book in Chemistry for JEE Main

Question

Pick out the correct statements of the following about liquids? (a) The intermolecular forces of attraction in a liquid are high. (b) All liquids suffer cooling on evaporation. (c) Lower the boiling point of a liquid, greater the vapour pressure at room temperature. (d) At higher altitudes water boils at a higher temperature than at the sea level.

Step-by-Step Solution

Answer

Correct statements are (a), (b), and (c).

Step 1: Evaluate Statement (a)

For liquids, intermolecular forces are stronger than those in gases but weaker than in solids. This allows liquids to maintain a definite volume but no fixed shape. Therefore, statement (a) is generally considered correct.

Step 2: Evaluate Statement (b)

Evaporation is an endothermic process, which means it absorbs heat from the surroundings, resulting in cooling. Thus, statement (b) is correct.

Step 3: Evaluate Statement (c)

A lower boiling point means that at a given temperature the liquid can easily vaporize, implying that the vapor pressure is higher. Therefore, statement (c) is correct.

Step 4: Evaluate Statement (d)

At higher altitudes, atmospheric pressure is lower, which causes water to boil at a lower temperature, not higher. Hence, statement (d) is incorrect.

Key Concepts

Intermolecular Forces

In liquids, intermolecular forces play a crucial role in determining properties like volume and shape. These are attractions between molecules that help keep the substance in the liquid state. While intermolecular forces in liquids are not as strong as in solids, they are much stronger than in gases. This is why liquids have a definite volume but can change shape, adapting to the container they are in. These forces include types such as hydrogen bonds, dipole-dipole interactions, and London dispersion forces. Each type contributes differently, influencing a liquid's boiling point and surface tension. Stronger forces generally lead to a higher boiling point because more energy is needed to overcome these forces.

Evaporation Process

Evaporation is a process where a liquid changes into a vapor, happening primarily at the surface. As a natural endothermic process, evaporation requires energy to take place. This energy is absorbed from the surroundings, which often leads to cooling of the surface from where the liquid evaporates. This is why, for example, sweating can cool your body as the sweat evaporates. Several factors can affect the rate of evaporation:

• Temperature: Higher temperatures increase the rate as more molecules have enough energy to escape into the vapor phase.
• Surface Area: More surface area allows more molecules to escape at any given time.
• Humidity: Lower humidity facilitates faster evaporation as the surrounding air can still accommodate additional vapor.

Boiling Point and Vapor Pressure

The boiling point of a liquid is the temperature at which its vapor pressure equals the surrounding atmospheric pressure. At this point, bubbles can form in the liquid, leading to boiling. Vapor pressure is linked to how easily molecules escape from the liquid. Liquids with lower boiling points have higher vapor pressure at room temperature because their molecules easily escape into the vapor phase with less energy. A well-known example is alcohol, which has a lower boiling point and therefore a higher vapor pressure than water at the same temperature. Understanding these concepts is key to explaining why some liquids evaporate and boil at different rates and temperatures.

Effect of Altitude on Boiling Point

At higher altitudes, the atmospheric pressure is lower than at sea level. This reduction in pressure affects the boiling point of liquids. Water, for example, boils at a lower temperature in a mountain environment compared to at sea level. This is because it requires less energy for the vapor pressure of water to match the reduced atmospheric pressure. As a result, cooking times can be longer at higher altitudes since the boiling point of water is lower, meaning foods may not be cooked at the usual temperatures people expect. This concept is crucial for understanding how changing environmental conditions can alter fundamental properties of liquids.